1. Field of the Invention
The present invention relates to generally to electromagnetic laser radiation by semiconductor lasers and, in particular, to vertical cavity surface emitting lasers (VCSELs) and methods for fabricating the same.
2. Description of the Related Art
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors, one of which serves as the “exit” mirror. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in a resonant cavity within the active region between the two mirrors. A portion of the coherent light built up in the resonant cavity passes through the exit mirror as the output laser beam.
Various forms of pumping energy may cause the active region to begin to emit photons. For example, semiconductor lasers of various types may be electrically pumped (EP) via a DC or alternating current, optically pumped, etc. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes current to flow therein. As a result of the applied potential, charge carriers (electrons and holes) are injected from opposite directions into the active region where recombination of electrons and holes occurs. Within the active region, two types of recombination events can occur simultaneously: radiative and non-radiative. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states. Some of those photons travel in a direction perpendicular to the mirrors of the laser. As a result of ensuing reflections between the two mirrors, the photons can travel through the active region multiple times.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. Particularly, stimulated emission occurs when a photon with an energy equal to the difference between an electron's energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon. Thus, when the photons produced by spontaneous electron emission interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If a sufficient amount of radiative recombinations are stimulated by photons, the number of photons traveling between the mirrors tends to increase, giving rise to amplification of light and lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes though the exit mirror as the output laser beam.
Semiconductor lasers may be classified as edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the substrate surface while SELs output their radiation perpendicular to the substrate surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The “vertical” direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with “up” being typically defined as the direction of deposition or growth. In some designs, the laser beam is emitted from the top side, in which case the upper of the two mirrors is the exit mirror. The exit mirror typically has a slightly lower reflectance, i.e., reflectivity, than the other, non-exit mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, and scalability to monolithic laser arrays. VCSELs have a shorter resonant cavity than edge-emitting lasers and thus have better longitudinal mode selectivity and narrower linewidths. Additionally, because the output is perpendicular to the substrate surface, it is possible to test fabricated VCSELs on the surface before extensive packaging must be done, in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for cleaving operations common to edge-emitting lasers.
The VCSEL structure usually consists of an active region sandwiched between upper and lower mirrors such as distributed Bragg reflectors (DBRs), i.e., top and bottom DBRs, respectively. Because the optical gain is low in a vertical cavity design, the DBRs require a high reflectance in order to achieve sufficient level of feedback for the device to lase. DBRs are typically formed of multiple pairs of layers referred to as mirror pairs. DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) layers, semiconductor layers, or a combination thereof including metal layers. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction; for semiconductor DBRs, the layers are typically selected so that they are easily lattice matched to other structures of the VCSEL, to permit epitaxial fabrication techniques.
When properly designed, mirror pairs will cause a desired reflectance at the lasing wavelength, at which wavelength the active region is also designed to have sufficient gain to permit lasing to occur. Typically in a VCSEL, the mirrors are designed so that the bottom DBR (i.e., the DBR interposed between the substrate material and the active region) has nearly 100% reflectance, while the top (exit) DBR has a reflectance that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top (exit) mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors.
For semiconductor DBRs, the number of mirror pairs per stack may range from 20-50 pairs to achieve a high reflectance, depending on the difference between the refractive indicies of the layers. As the number of mirror pairs increases, the percentage of reflected light increases. The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectance to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in dielectric DBRs, a smaller number of mirror pairs can achieve the same reflectance as a larger number of mirror pairs in semiconductor DBRs. However, despite their lower reflectance/greater thickness, semiconductor DBRs can be preferred because of comparative advantages in electrical conductivity, thermal conductivity, and manufacturability. For example, in an EP VCSEL, semiconductor DBRs are preferred, especially as bottom DBRs (between the substrate and active region), to conduct electrical current through the active region, the bottom DBR, and into the substrate. Accordingly, there is often a tradeoff between using a lower reflectance, thicker semiconductor DBR, or a higher reflectance, thinner dielectric DBR which is more difficult to manufacture or which makes thermal conductivity more of an issue.
While generally successful, VCSELs have problems. For example, a major problem in realizing commercial quality long wavelength VCSELs (e.g., VCSELs lasing at wavelengths longer than 1000 nm such as 1300 nm to 1550 nm) is the available mirror materials. Long wavelength VCSELs are of great interest in the optical telecommunications industry because of minimum dispersion in fibers at 1320 nm and minimum loss in fibers at 1550 nm. Long wavelength VCSELs are often based on InP material systems. For proper lattice matching, an InP-based VCSEL usually uses InP/InGaAsP or AlInAs/AlInGaAs DBRs. However, because those materials have relatively small differences in refractive indicies, an unacceptable number of mirror pairs (e.g., 40-50) are typically needed to achieve the required reflectance. Growing that number of mirror pairs takes a long time, which increases the production costs. Moreover, as the operating (emitting) wavelength of the laser increases, the thickness of each layer within the mirror pair must also increase, further contributing to the increased thickness of long-wavelength semiconductor DBRs.
Long-wavelength VCSELs thus would require a comparatively thick semiconductor bottom DBR, which can be difficult to manufacture. Such thick DBRs can also have poor thermal conductivity, so that it is difficult to achieve adequate heat dispersion to the heat-spreading submount to which the bottom DBR is mounted. The formation of the thick semiconductor DBR on an InP substrate, for example, causes serious manufacturability problems, as described above.
Many attempts have been made to address this problem, including fabrication of devices via wafer bonding techniques, but only limited success has been achieved. As an example, VCSEL devices can be formed by independently growing a semiconductor DBR on a GaAs substrate and an active layer on an InP substrate. The two components are then flip-chip mounted together and fused using wafer fusion techinques. The process described above, however, yields a device that is expensive to manufacture and exhibits low efficiency, low output power, and low yield. In addition, the interface defect density in the wafer fusion procedure causes potential reliability problems of the VCSEL end product.